1. Field of the Invention
The present invention relates to a red phosphor, and more particularly to a red phosphor with high emission efficiency, a method of preparing the red phosphor, and a red light emitting diode, a white light emitting diode, and an active dynamic liquid crystal device using the red phosphor.
2. Description of the Related Art
White light emitting diodes (LEDs) have a longer lifetime than incandescent light bulbs (a popular type 60 W) can be manufactured in small sizes, and operated at a low voltage. Therefore, white LEDs are recognized as an alternative light source in areas such as domestic fluorescent lamps, liquid crystal device (LCD) backlights, and the like.
White LEDs can be manufactured using red, green and blue LEDs. In this case, however, the white LEDs have a high manufacturing cost, and are bulky due to complex driving circuits. A white LED manufactured by combining a InGaN-based blue LED having a wavelength of 450 nm and a YAG:Ce phosphor has been put to practical use. The white LED is operated by performing the following steps. First, the blue LED emits blue light. A YAG:Ce phosphor is excited by a part of the blue light, emitting a fluorescent yellowish green light. Then, the blue light and the fluorescent yellowish green light combine to form white light. However, this white light has only a portion of the visible range spectrum, which results in a small color rendering index and poor color rendition. In addition, since the blue LED, which is used as a light source in this case, has a wavelength of 450 nm, chip efficiency is low and thus the emission efficiency of the white LED deteriorates.
In order to solve these problems, a UV LED is used as a light source for excitation and a combination of a red phosphor, a green phosphor, and a blue phosphor is used in the manufacturing process of a white LED to obtain nearly natural white light. However, above all, a phosphor that exhibits high emission efficiency when excited by an excitation light source of about 410 nm where chip efficiency is highest is an essential prerequisite.
Currently, conventional blue phosphors and green phosphors have good emission efficiency. However, a red phosphor that exhibits high emission efficiency when excited by a UV excitation light source must be developed as soon as possible since red phosphors have poor emission efficiency.
In addition, the introduction of the phosphor that exhibits high efficiency when excited by long-wavelength UV can facilitate the improvement of an active dynamic LCD. In the active dynamic LCD, light emitted from a rear light source passes through a polarizer and then through a liquid crystal layer. The liquid crystal layer transmits or does not transmit the rear light depending on its orientation. As a result, the rear light is polarized in a predetermined manner. This polarized light excites a corresponding phosphor which then emits light. Thus, an image is formed on a front glass substrate. The active dynamic LCD has a simple structure and can be easily manufactured compared to conventional color LCDs. However, the active dynamic LCD is not commercially available due to low emission brightness of conventional red phosphors. In particular, in the active dynamic LCD, the rear light source must be long-wavelength UV of 390 nm or greater to protect the liquid crystal. A competitive candidate for the rear light source is the UV LED. Therefore, the development of a red phosphor that exhibits high efficiency when exited by long-wavelength UV light will contribute to the development of the active dynamic LCD as well as to the improvement of the red LED and the white LED.
Currently, 3.5MgO0.5MgF2GeO2:Mn, K5Eu(WO4)6.25 is used as the red phosphor for long-wavelength UV. However, 3.5MgO0.5MgF2GeO2:Mn, K5Eu(WO4)6.25 exhibits low emission brightness, and low emission efficiency when excited by an excitation light source of 400 nm or greater.